Specifies whether the element represents an element that
is is focusable (that is, an element which is part of the
sequence of focusable elements in the document), and the
relative order of the element in the sequence of focusable
elements in the document.

The UA can resume playback of media data for this
video or audio element, but estimates that if playback
were to be started now, the video or audio could not be
rendered at the current playback rate up to its end
without having to stop for further buffering of
content.